drm/i915/fence: Use rcu to defer freeing of irq_work

It is illegal to perform an immediate free of the struct irq_work from
inside the irq_work callback (as irq_work_run_list modifies work->flags
after execution of the work->func()). As we use the irq_work to
coordinate the freeing of the callback from two different softirq paths,
we need to defer the kfree from inside our irq_work callback, for which
we can use kfree_rcu.

Fixes: 81c0ed21 ("drm/i915/fence: Avoid del_timer_sync() from inside a timer")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213094802.28243-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_sw_fence.c

@@ -367,6 +367,7 @@ struct i915_sw_dma_fence_cb {
 	struct dma_fence *dma;
 	struct timer_list timer;
 	struct irq_work work;
+	struct rcu_head rcu;
 };
 
 static void timer_i915_sw_fence_wake(struct timer_list *t)
@@ -406,7 +407,7 @@ static void irq_i915_sw_fence_work(struct irq_work *wrk)
 	del_timer_sync(&cb->timer);
 	dma_fence_put(cb->dma);
 
-	kfree(cb);
+	kfree_rcu(cb, rcu);
 }
 
 int i915_sw_fence_await_dma_fence(struct i915_sw_fence *fence,